1. Field of the Invention
The present invention relates to a refrigerating system of a vapor compression type and is suitable for a refrigerating system where a cooling medium, such as a carbon dioxide (CO.sub.2) is used in a super critical state.
2. Description of Related Art
It has, recently, been strictly required that a refrigerant other than a fluorocarbon (flon) be used in a refrigerating system. Thus, a vapor compression type refrigerating system has been proposed in which carbon dioxide (CO.sub.2) is used as a refrigerant in order to execute a refrigerating cycle. See, for example, Japanese Examined Patent Publication (Kokoku) No. 7-18602. This type of a refrigerating system is referred to below as a CO.sub.2 refrigerating system.
This CO.sub.2 refrigerating system operates, basically, in the same manner that a conventional refrigerating system, using flon as a refrigerant does. However, the CO.sub.2 refrigerating system is defective in that a pressure at the outlet of the heat emitter is increased to a value as much as 10 times of that in the conventional refrigerating system using flon when a refrigerating capacity is high due to an increased outside air temperature such as that in a summer season. Such an increase in the pressure at the outlet of the heat emitter means that the strength of parts constructing the compressor must be correspondingly increased over those in the conventional refrigerating system using flon, resulting in an increase in the size of the system.
Furthermore, in the CO.sub.2 refrigerating system, the pressure and the temperature of the CO.sub.2 at the outlet of the heat emitter are controlled along a so-called optimum control line, which makes the coefficient of performance of the system attain the maximum value. However, the coefficient of performance is calculated as a ratio of the change in an enthalpy at the evaporator to compression work at the compressor. Thus, such control along the optimum control line does not necessarily guarantee a desired refrigerating capacity at a particular condition, such as a cool-down operation.
Furthermore, in the CO.sub.2 refrigerating system, the pressure reducer is controlled in such a manner that the pressure at the outlet of the heat emitter is varied in accordance with the temperature of the CO.sub.2 at the outlet of the heat emitter. However, such a control may cause the degree of super-heating of the refrigerant to be insufficient at the outlet of the evaporator, which may cause a liquid state refrigerant to be sucked by the compressor, thereby damaging the compressor. On the other hand, in a case where the degree of the opening of the pressure reducer is controlled such that a desired degree of the superheat at the outlet of the evaporator is obtained, a relationship between the pressure at the outlet of the heat emitter and the temperature at the outlet of the heat emitter deviates from the optimum control line, thereby reducing an efficiency of a refrigerating cycle using CO.sub.2.